I was looking around a little and it is possible to buy GRC on the BitShares market and pay the fees with a different currency than BTS, but only with GRC.

Under the field for the Fee on the purchase or sale order you can switch BTS for GRC.

I know this doesn't solve your initial issue with buying the GRC in the first place without any BTS, but you can now after your first purchase pay the fees in GRC.

Fee Pool

How it works is that the Issuer of an asset has a fee pool that pays the fees for you at a current exchange rate equal to that in BTS. This is up to the issuer to have.

Source: https://steemit.com/bitshares/@xeroc/bitshares-fee-schedule